A pH-sensor for registering the pH-value of a medium ordinarily comprises two coaxial glass tubes, with the outer glass tube being connected with the inner glass tube at the media-contacting end, so that the media-side opening of the outer glass tube is sealed, and with the media-contacting end of the inner glass tube having an ion-sensitive glass membrane. In the media-contacting end section of the outer tube there is additionally provided at least one diaphragm, via which an electrolyte bridge is assured between a reference liquid in an outer chamber, which is formed between the inner glass tube and the outer glass tube, and a medium surrounding the sensor.
Manufacture of the glass body involves much manual work and is very complex. The assignee has automated a method for manufacturing the inner glass tube having an ion-sensitive, glass membrane blown thereon, as described in German Patent No. DE 10 116 075.5 and Offenlegungsschrift No. DE 10 116 099. Inner tubes manufactured in such manner are combined with an outer glass tube into a so-called single-rod, measuring chain by inserting a drilled-through, porous plug of PTFE into the media-side end of the outer glass tube and by inserting the inner glass tube into the drilled bore of the porous stopper. There is, in contrast, still no automatic fabricating process for the glass body of a single-rod measuring chain, including the ion-sensitive membrane with a glass connection between the inner glass tube and the outer glass tube. The reasons therefor are two-fold. First, a subsequent fusion joining of the end region of an outer glass tube with the end region of an inner glass tube, when the inner glass tube already has been equipped with an ion-sensitive membrane, would endanger the ion-sensitive membrane. Second, it has not yet been possible to connect the outer glass tube with the inner glass tube in such a manner that, on the edge of the remaining opening of the inner glass tube, an ion-sensitive glass membrane can be blown using an automatic process according to the above-referenced intellectual property.